Here is a wall which I carved. I used plain old plaster of paris and the build was in 1/2" scale (1/25th)

The metal is a piece of aluminum which is spring loaded that I used to lay out the bricks before carving. It held the plaster wall in place. The only other tools was a 6" rule and the carving device.
This is my practice piece after finishing.

The whole process was sort of fun, but there is always the realization in the back of your mind that if you screw up, well........
O scale would be the smallest I would attempt. Tried HO once. My eyes couldn't handle it.
Marc's idea about buying some walls is a good one, but doing it yourself will be more satisfying when the project is complete. Just my 2 cents...